A nutcase. As in not right in the head. Always have their shirt tucked into their trousers which are half way up their chest.
A person who is most likely to come and regail you with stories in a pub of when they "Nutted this bloke" and how his helper is nicking money from him.
They vary in condition, some are worse than others, with the worst ones not having control of bladder. A common hobbie of a Jellyhead is shouting at passing cars.

Best avoided
"Look at that Jellyhead shouting at the traffic"
"Why's that Jellyhead walking about with his trousers round his ankles?"
by colujimbo November 8, 2006
Get the Jellyhead mug.
John: Hey Lisa, your favorite color is purple right?
Lisa: Yeah, why?
John:Because I got some purple jellyheads we could use tonight
Lisa:Oooh really? I love the grape flavored ones!
by Gizzly Adamz September 18, 2009
Get the Jellyhead mug.